In the pulping of comminuted cellulosic fibrous material, such as wood chips, in the continuous digester the material is treated to remove entrapped air and to impregnate the material with cooking liquor while raising its pressure and temperature (e.g. to 150.degree. C. and 165 psi). Typically, the chips are steamed to purge them of air while simultaneously increasing their temperature, passed through air locks to raise their pressure, impregnated with heated cooking liquor, and then transported as a slurry to the digester.
In the past, in order to accommodate the purging, heating, pressurizing, and feeding functions, an apparatus is provided that is bulky, tall, and expensive. Normally a special building or super structure must be built to house or support this equipment. Such a building or super structure is built with structural steel and concrete, requires utilities, stairwells, and other accouterments, and contributes greatly to the cost of a continuous digester system. Also, the cost of the conveyor which transports chips to the inlet to the system is highly dependent upon the overall height of the system, which is typically on the order of about 115 feet for a digester which has a capacity of about 1,500 tons per day.
According to the present invention a system is provided for delivering a slurry of comminuted cellulosic fibrous material to a continuous digester that has numerous advantages compared to the prior art. According to the present invention, the delivery system is much less massive, tall, and expensive than the conventional systems. For example, the system according to the present invention may have a height of only about 60 feet for the same size digester that the prior art systems would have a height of 115 feet. Also, the system according to the present invention has a higher delivery capacity--that is, for a particular size of equipment, it can deliver more slurry to the top of the digester per unit time. Because of the much smaller size of the system according to the present invention, the prior art building or super structure can be eliminated or downsized so that it is significantly more economical, leading to a complete system which is much less expensive than prior art systems.
In the conventional delivery systems, the high pressure feeder, which is a high pressure rotary transfer device such as shown in U.S. Pat. No. 4,372,711, is mounted on an elevated concrete pedestal. Such a mounting is necessary because the draw-through system used for pulling chips from a chip chute through the high pressure feeder requires a minimum static head to operate effectively. The chip bin is typically a large cylindrical vessel, and it is connected by a chip feeder and a low pressure feeder to a horizontal steaming vessel, which in turn is connected to a vertical generally cylindrical superatmospheric pressure chip chute connected to the top of the high pressure feeder. The recirculation line, which includes a low pressure pump mounted below the high pressure feeder, includes a superatmospheric pressure level tank which controls the level of liquid in the chip chute.
According to the present invention, virtually every element of the delivery system, except for the high pressure feeder itself, is modified so as to reduce the height and bulk of the equipment, and in one case to also increase the effective capacity of the high pressure feeder.
According to one aspect of the present invention, which has the greatest single affect in minimizing the height, and simultaneously increasing the effective capacity of the high pressure feeder, a modification to the low pressure circulation line associated with the high pressure feeder is provided. Instead of the chip chute on top of the high pressure feeder and the chip chute pump below the high pressure feeder, providing a "suck through" system, a pump-through system is provided according to this aspect of the present invention. According to this aspect of the invention a system for delivering chip slurry to the continuous digester comprises: A high pressure rotary transfer device having a low pressure inlet, low pressure outlet, high pressure inlet, and high pressure outlet, the high pressure outlet operatively connected (e.g., directly, through an impregnation vessel, or the like) to a continuous digester for feeding comminuted cellulosic fibrous material slurry to the digester. A vessel at substantially atmospheric pressure containing a slurry of comminuted cellulosic fibrous material, and having a top, a bottom, and an outlet adjacent the bottom. A slurry pump connected between the vessel outlet and the transfer device low pressure inlet. And, a recirculation loop for returning liquid from the transfer device low pressure outlet to the vessel. The vessel, slurry pump, and high pressure transfer device are typically mounted substantially at ground level. That is, one need not be mounted on top of the other, and no concrete pedestal is necessary to mount the high pressure feeder.
The recirculation loop of the system according to the invention typically includes an in-line drainer connected to a substantially atmospheric pressure level tank for controlling the level of slurry in the vessel. In order to avoid water hammer due to flashing of liquid in the high pressure feeder, a means for lowering the temperature of the recirculating liquid in the recirculation loop, such as a liquid cooler (indirect heat exchanger), or a vessel which allows the liquid to flash, is provided. Temperature sensors can be provided on opposite sides of the heat exchanger, and a controller can provide for controlling the flow of coolant through the heat exchanger in response to the temperature sensors. The temperature of the liquor in this return recirculation can also be controlled by cooling the white liquor before adding it. Similar methods to those used in U.S. Pat. No. 5,302,247 may be used to cool the white liquor. This white liquor cooling may be controlled based on the temperature sensed at upstream temperature sensor.
The system can also include a second (or even more) high pressure rotary transfer device which is fed by the same slurry pump. A flow control valve may be provided in the recirculation loop with pressure sensors for sensing the pressure between the slurry pump and the transfer device low pressure inlet, and the pressure in the recirculation line, controlling the flow control valve in response to the pressure sensors.
By utilizing the pump-through feed of chips as described above, the height of the chip delivery system can be reduced about 20-30 feet, with a commensurate simplification of associated equipment. The system also allows the high pressure feeder to run faster, and allows more than one feeder to be run in parallel simplifying the design of new systems and increasing the capacity of existing systems. In a conventional draw-through design, the suction of the chip chute pump reduces the pressure at the bottom of the feeder. When slurry is at a temperature greater than 220.degree. F. (a typical slurry temperature at the high pressure feeder is about 240.degree.-260.degree. F.) the reduction of pressure can cause flashing of the hot liquor and thus water hammer. The potential for inducing flashing increases as the speed of the feeder increases by causing increased pressure drop. The potential for inducing water hammer presently limits the speed at which conventional high pressure feeders can be operated. (Some feeders are typically limited to 11 rpm.) In the pump-through system according to the invention, since there is no suction at the liquor outlet, the potential for inducing water hammer is minimized, if not eliminated. Thus the high pressure feeder can be operated at higher speeds and increased capacity, allowing smaller units to be used in new systems, and allowing existing high pressure feeders to run at higher speeds and increased capacity.
The pump-through design also has the potential to increase the feeder capacity by allowing higher flows. As discussed above, flow in the chip chute circulation, i.e., from the chip chute, through the feeder, through the chip chute pump, etc. is limited due to pressure drop across the feeder and the potential for flashing. Since the potential to flash in the feeder is minimized in the pump-through system, higher liquor flows can be achieved without flashing. These higher liquor flows through the feeder will aid in filling the feeder pockets with chips, hence increasing the feeder's capacity.
The pump-through design also improves the efficiency of systems that may contain air or entrained gases in the chip chute slurry. The presence of air, or other gases, in the chip-liquor slurry reduces the flashing temperature of the hot liquor. Where liquor under 15 psig pressure may flash at 250.degree. F., liquor containing trapped air under 15 psig may flash at somewhat lower temperatures, e.g., 230.degree. F.
The pump-through system and the push-through system (i.e., the system with the pressurized chip chute and atmospheric level tank) are advantageous when air is present because the low-pressure areas, that create flashing, do not occur in and around the high-pressure transfer device. In the pump-through design, the low pressure area is in the atmospheric chip chute pump impeller. In the push-through system, the low-pressure area is in the atmospheric level tank where flashing can be beneficial to produce steam for pre-steaming.
According to another aspect of the present invention, the height of the delivery system is further significantly reduced by utilizing--in place of the conventional cylindrical chip bin--a hopper having two transitions with one dimensional convergence and side relief. The general design of such a hopper is shown in U.S. Pat. No. 4,958,741 (the disclosure of which is hereby incorporated by reference herein), and detailed configurations suitable for use as chip bins are shown in co-pending application Ser. No. 08/189,546 filed Feb. 1, 1994, the disclosure of which is hereby incorporated by reference herein. By utilizing the hopper with one dimensional convergence in place of the conventional cylindrical chip bin a height reduction on the order about 15 feet can be obtained.
According to another aspect of the present invention, with the new chip chute pump providing the motive force which fills the feeder, the intermediate pressure raising devices of conventional delivery systems can be eliminated. This can be done by operating the chip chute (vessel) at substantially atmospheric pressure (e.g. 1 bar or slightly above), which is connected directly to the chip bin without pressure isolation. That is, the low pressure feeder is eliminated, reducing the height of the delivery system by about five feet.
The height of the delivery system may be reduced even further by replacing the conventional chip chute with a vessel having one dimensional convergence and side relief, such as shown in U.S. Pat. No. 4,958,741. This reduces the height another five to ten feet, approximately.
Utilizing all of the modifications as set forth above, it is possible to provide a delivery system that has a height only 40-50% of conventional systems, without the necessary complex super structure (with associated stairwells, utilities, and the like), concrete pedestal for supporting the high pressure feeder, and the like. For example, instead of a 115 foot high delivery system which is typical for use with a 1,500 ton per day continuous digester (with or without impregnation vessel), a delivery system having a height of about 60 feet may be provided.
Other modifications may be provided too. For example according to another aspect of the present invention a system for delivering slurry to a continuous digester includes the following components associated with the high pressure transfer device: A vessel at superatmospheric pressure containing a slurry of comminuted cellulosic fibrous material, and having a top, a bottom, and an outlet adjacent the bottom. A chip bin mounted above the vessel and connected to the vessel by a low pressure feeder for feeding cellulosic fibrous material to the vessel at superatmospheric pressure. A recirculation loop for returning liquid from the transfer device low pressure outlet to the vessel. And, a substantially atmospheric pressure level tank disposed in the recirculation loop for controlling the level of slurry in the vessel, and a pump between the vessel and the level tank for pressurizing liquid and pumping it from the level tank to the vessel. The transfer device is preferably mounted substantially at ground level. The chip bin is preferably as described above. Also a steam conducting conduit is preferably provided for transporting steam from the liquid flashing in the atmospheric pressure level tank to the chip bin.
One advantage of using an unpressurized, atmospheric level tank is that a larger tank is practical. The present pressurized level tank is limited in size due to the cost of designing and fabricating a larger vessel which meets ASME (i.e. American Society of Mechanical Engineers) pressure vessel design codes. A larger, unpressurized vessel can be built more cheaply. A large, unpressurized level tank would also better control and accommodation of both short- and long-term variations, i.e. "swings", in system operation. Short-term swings include variation in digester production rate and variation in chip feed. Long-term swings include variations in chip moisture or chip volume. Make-up liquor flow from a large level tank to the digester can be controlled by monitoring the pressure in the digester.
According to yet another aspect of the present invention a system for delivering slurry to a continuous digester, in addition to the high pressure transfer device, comprises: A vessel at substantially atmospheric pressure containing a slurry of comminuted cellulosic fibrous material, and having a top, a bottom, and an outlet adjacent the bottom. A substantially atmospheric pressure chip bin mounted above the vessel and connected directly to the vessel without pressure isolation. A recirculation loop for returning liquid from the transfer device low pressure outlet to the vessel. And, a substantially atmospheric pressure level tank disposed in the recirculation loop for controlling the level of slurry in the vessel.
The invention also comprises a comminuted cellulosic fibrous material treatment system. The treatment system includes: A continuous digester having a comminuted cellulosic fibrous material inlet adjacent the top thereof. And, a combination of elements for feeding material slurry to the digester, the combination comprising: a high pressure rotary transfer device having a low pressure inlet, low pressure outlet, high pressure inlet, and high pressure outlet, the high pressure outlet operatively connected to a continuous digester for feeding comminuted cellulosic fibrous material slurry to the digester; a vessel containing a slurry of comminuted cellulosic fibrous material, and having a top, a bottom, and an outlet adjacent said bottom; a chip bin mounted above the vessel and connected to the vessel for feeding cellulosic fibrous material to the vessel; a recirculation loop for returning liquid from the transfer device low pressure outlet to the vessel; and a level tank disposed in the recirculation loop for controlling the level of slurry in the vessel. And, the combination of elements having a maximum height which is less than about 35% of the height of the digester.
Utilizing the system described above, a method of delivering a slurry of chips to the continuous digester (either through an impregnation vessel, or directly to the top of the digester) is provided which allows operation of the high pressure transfer device at a significantly higher operating speed than conventional, e.g. at operating speeds of about 15 rpm or higher, with a commensurate increase in capacity.
It is the primary object of the present invention to provide a less costly, improved, delivery system for delivering comminuted cellulosic fibrous material slurry to a continuous digester. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.